Conventionally, a mask to be used in a pattern exposure apparatus when producing a semiconductor device or the like is a transmissive mask, a reflective mask for Extreme Ultra Violet (EUV) exposure, or the like. In a case where a defect or the like is present in a pattern of such a mask, there has been known a method in which not an entire surface of an expensive mask is produced again, but only a defect or the like is repaired from economical and temporal points of view. A repair apparatus to be used for this purpose can also be applied to a nano-imprint original plate (mold) other than the mask, and there has been known a mask repair apparatus using a focused ion beam (FIB) of gallium ions or an electron.
For example, there has been disclosed an apparatus using as an irradiation beam, a focused ion beam formed by focusing ions emitted from a liquid metal ion source in an ion optical system (for example, JP-A-2003-156833). This apparatus repairs a mask by removing an unnecessary pattern in a minute region and depositing a film onto a pattern missing portion or the like using a focused ion beam of gallium ions focused to a diameter of about several nanometers.
For example, there has been disclosed a mask repair apparatus using an electron beam and an assist gas (for example, JP-A-2003-328161). In this apparatus, as compared with the focused ion beam of gallium ions, a decrease in transmittance due to ion implantation is prevented, and the apparatus has excellent beam focusing performance.
Further, conventionally, there has been known a gas field ion source (GFIS) as an ion source of a focused ion beam apparatus which is expected to have better focusing performance than the focused ion beam of gallium ions. The gas field ion source can form a focused ion beam by ionizing a source gas in a high electric field region at the apex of a tip, and the degree of energy spread of emitted ions is as small as a fraction of that of the liquid metal ion source, and therefore, the beam focusing performance thereof is higher than the focused ion beam of gallium ions. In the same manner as an electron beam, an etching effect is exhibited by secondary electrons and an assist gas, however, a region where the secondary electrons are generated is smaller than in the case of an electron beam, and therefore, it is expected that processing in a minute region can be performed.
Incidentally, a sharpened needle-like electrode which generates ions in a gas field ion source is referred to as a “tip”. This sharpened needle-like electrode is also used as a needle-like electrode of an electron source in an electron microscope, a probe in a scanning probe microscope, and the like. Conventionally, in order to obtain a high resolution image in an electron microscope and a focused ion beam apparatus, the apex of a tip has been desired to be sharpened in a level of several atoms. Further, in order to prolong the lifetime of a tip and also to obtain a high resolution image in a scanning probe microscope, it has been demanded that the apex of a tip should be sharpened to an atomic level, and the lifetime should be prolonged.
FIGS. 13A to 13C show schematic shapes of a conventional tip 500. As shown in FIG. 13A, the tip 500 is formed such that the tip end of a fine wire having a diameter of several hundreds of micrometers or less has a narrow and sharp shape by electrolytic polishing (also referred to as “wet etching”). As show in FIG. 13B, the tip 500 has a minute protrusion 501 in the apex portion B. As shown in FIG. 13C, the minute protrusion 501 has a triangular pyramid shape formed by stacking several atomic layers, and the apex of the protrusion 501 is constituted by at most several atoms. Hereinafter, the protrusion 501 is referred to as a “pyramid structure”.
Next, the principle of ion generation of the gas field ion source using the tip 500 will be described with reference to FIG. 14.
As an ion source, a gas to be ionized is supplied, and in the surroundings of the tip 500, a gas molecule or atom (hereinafter described for short as “gas molecule”) 601 to be ionized exists. The tip 500 is cooled by a cooling device (not shown).
When a voltage is applied between the tip 500 and an extraction electrode 603 by a power source 602, and a high electric field is generated around the apex of the tip 500, the gas molecule 601 drifting around the tip 500 is polarized and moves as if it is attracted to the apex of the tip 500 by the polarizing power. Then, the attracted gas molecule 601 is ionized by the high electric field at the apex of the tip 500.
The generated ion 604 is emitted from an opening 603a of the extraction electrode 603 to a sample (not shown) through an ion optical system (not shown) downstream of the opening. Since the size of a region from which a beam of the ions 604 (ion beam) is emitted, that is, the source size of the ion source is extremely small, this gas field ion source becomes a high brightness ion source and can form an extremely fine focused ion beam on the sample.
Conventionally, there has been known a technique using a tungsten tip in a focused ion beam apparatus (or an ion microscope) including a gas field ion source (JP-A-2009-517840).
For example, there has been disclosed a method for repairing a defect in a photomask using a gas field ion source (for example, WO 2009/022603). In a repair apparatus using a rare gas ion beam generated from this gas field ion source, a decrease in mask transmittance (for example, a decrease in mask transmittance due to implantation of gallium in the mask caused by a conventional focused ion beam of gallium ions) can be reduced. Further, argon which is a kind of rare gas has a larger mass than an electron, and therefore, the processing efficiency is increased as compared with the case of repairing a mask using an electron beam.
There has been known, for example, a mask repair apparatus which forms a focused ion beam by ionizing nitrogen (for example, JP-A-2013-89534). This mask repair apparatus uses, as the tip, a tip composed of tungsten or molybdenum, or a tip obtained by coating a needle-shaped base material composed of tungsten or molybdenum with a noble metal such as platinum, palladium, iridium, rhodium, or gold.
There has been disclosed, for example, repair of a mask for extreme ultraviolet light exposure using a hydrogen ion beam generated from a gas field ion source (for example, JP-A-2011-181894). A tip for this mask repair is formed by coating a needle-shaped base material composed of tungsten or molybdenum with a noble metal such as platinum, palladium, iridium, rhodium, or gold. The apex of this tip is in a pyramid shape sharpened at the atomic level.
A method for sharpening the apex of a tip at the atomic level is important, but it is difficult to carry out the method, and various methods have been known.
In a tip, a crystal plane with a low planar atomic density on the crystal surface tends to be sharpened, and therefore, a tungsten tip is sharpened in the <111> direction. The {111} crystal plane of tungsten has a threefold rotational symmetry, and a {110} crystal plane or a {112} crystal plane composes a side surface (pyramid surface) of a pyramid structure.
As a method for sharpening the apex of the tungsten tip in a level of several atoms, there has been known a method such as field-induced gas etching using nitrogen or oxygen, thermal faceting, or remolding, and by those methods, the apex in the <111> direction can be sharpened with high reproducibility.
The field-induced gas etching is a method for etching a tungsten tip by introducing nitrogen gas while observing a Field Ion Microscope (FIM) image using helium or the like as an imaging gas by an FIM. The ionization field strength of nitrogen is lower than that of helium, and therefore, nitrogen gas cannot come closer to a region where the FIM image can be observed (that is, a region where helium is field-ionized) and is adsorbed on the side surface of the tip at a short distance away from the apex of the tungsten tip. Then, the nitrogen gas is bonded to a tungsten atom on the surface of the tip to form tungsten nitride. Tungsten nitride has low evaporation field strength, and therefore, only the side surface of the tip at a short distance away from the apex on which nitrogen gas is adsorbed is selectively etched. At this time, a tungsten atom at the apex of the tungsten tip is not etched, and therefore, a tip having a further sharpened apex than an electrolytically polished tip can be obtained (for example, U.S. Pat. No. 7,431,856B).
The thermal faceting is a method for forming a polyhedral structure at the apex of a tip by heating the tip after electrolytic polishing in an oxygen atmosphere to grow a specific crystal plane (for example, JP-A-2009-107105).
The remolding is a method for forming a crystal plane at the apex of a tip by heating and applying a high voltage to the tip after electrolytic polishing under ultrahigh vacuum conditions (for example, JP-A-2008-239376).
Further, there has been known a scanning ion microscope (i.e. a focused ion beam apparatus) using a focused helium ion beam which includes a gas field ion source using a tungsten tip sharpened at the atomic level (for example, William B. Thompson et al., Proceedings of the 28th Annual LSI Testing Symposium (LSITS 2008), (2008) pp. 249-254, “Helium Ion Microscope for Semiconductor Device Imaging and Failure Analysis Applications”). In this focused ion beam apparatus, the apex of the tip is constituted by three tungsten atoms (a trimer), each of which emits ions, and ions emitted from one atom among these three atoms are selected and focused into a beam.
Further, it has been known that in a scanning ion microscope using a focused helium ion beam which includes a gas field ion source using a tungsten tip, the apex of the tungsten tip which emits ions is terminated with a trimer composed of three tungsten atoms, each of which emits ions, and ions emitted from one atom among these three atoms are selected and focused into a beam (for example, B. W. Ward et al., Journal of Vacuum Science & Technology, vol. 24, (2006), pp. 2871-2874, “Helium ion microscope: A new tool for nanoscale microscopy and metrology”).
Further, it has been known that a minute triangular pyramid structure composed of one {110} crystal plane and two {311} crystal planes is formed at the apex of an iridium<210> single crystal tip (for example, Ivan Ermanoski et al., Surf. Sci. vol. 596, (2005), pp. 89-97, “Atomic structure of O/Ir(210) nanofacets”).
Further, it has been known that a minute triangular pyramid composed of one {110} crystal plane and two {311} crystal planes is formed by thermal faceting at the apex of a sharpened iridium<210> single crystal tip, and the apex thereof is constituted by a single atom. A gas field ion source using this iridium tip can continuously emit a beam for about 2,250 seconds (see, for example, Hong-Shi Kuo et al., Nanotechnology, vol. 20, (2009) No. 335701, “A Single-atom sharp iridium tip as an emitter of gas field ion sources”).